An optical interconnection device disclosed in a paper by M. Tabiani and M. Kavehrad entitled "Theory of an efficient N.times.N passive star coupler", Journal of Lightwave Technology, Vol. LT-9, No.4, pp. 448-455, April 1991, and in International patent application No. PCT/CA 91/00113, both of which are incorporated herein by reference, and U.S. patent application Ser. No. 594,137, now U.S. Pat. No. 5,179,605, which is incorporated by reference and appended hereto, comprises a stratified volume BRAGG diffraction means with a spatially varying refractive index. The volume diffraction means comprises gratings which are three-dimensional periodic structures which couple planar light waves from N input fibers to N' output fibers, the prime signifying that the number of output fibers could be different from the number of input fibers. The gratings may be provided using holographic techniques.
In the coupler disclosed in PCT/CA 91/00113, the refractive index of the diffraction means varies spatially according to the expression: ##EQU1## where x and z are two of the three spatial ordinates of the grating medium;
k.sub.m,m' is the spatial frequency vector; PA1 m is an input position or mode, corresponding to one optical axis; PA1 m' is an output position or mode, corresponding to one optical axis; PA1 m and m' take integer values that determine the number of input/output modes; PA1 .DELTA..sub.m,m' is the modulation index which determines the coefficient of coupling between m and m'; PA1 r is the space vector (x,y,z). (Neglecting y gives a two-dimensional arrangement.)
Diffraction theory shows that, when N'.gtoreq.N, such configurations can potentially achieve substantially 100 per cent efficiency. In such a coupler, when N=N', the number of different gratings in the diffraction means is 1/2(N.sup.2 -N). This number increases approximately with the square of the number of inputs N. Thus, while a 3.times.3 coupler requires 3 gratings, a 10.times.10 coupler requires 45 gratings. The dynamic range of the refractive index of the slab, however, can be shown to increase with N.sup.3/2. For a practical holographic recording material, for example Dichromated Gelatin (DCG), which has a dynamic range less than 0.1, the dynamic range of the recording material will set a limit on the number of users or inputs N. The problem is compounded by the fact that high dynamic range (high modulation index) and low noise are usually conflicting requirements.
An object of the present invention is to mitigate this problem and provide an improved optical interconnection device.